The present invention relates, in its most general sense, to the production and/or recovery of industrially useful products from waste sludges containing calcium carbonate and at least one silicate mineral (e.g. kaolin). More specifically, the invention relates to the production and/or recovery of industrially useful products from an ash produced by incineration of the sludge. One aspect of the invention relates to the production of calcium carbonate from the sludge via its conversion to the ash. A further aspect of the invention is the recovery of silicate minerals from such an ash which are relatively free of lime.
The invention is concerned particularly (but not exclusively) with a method for the production and/or recovery of useful materials from waste sludge produced by a paper mill, and more particularly by a paper mill using recycled paper as a raw material, the method involving production of an ash from the sludge as an “intermediate”. Of prime concern is the production from such sludge of calcium carbonate for use in paper, polymers, coatings and sealants where high value calcium carbonate products are required while a secondary concern is the recovery and beneficiation of metakaolin for use in cement, concrete, polymers and coatings.
Calcium carbonate is the main mineral pigment used in paper manufacturing both as a filler and as a coating material. Calcium carbonate is also used extensively as a functional filler in materials such as paints, coatings, plastics, sealants and inks. Other applications of calcium carbonate are in the food, cosmetics and pharmaceutical industries
For paper coating the manufacturer needs a pigment which gives good optical properties (high brightness, opacity and gloss) and good printability. The morphology of the pigment is important to give the appropriate rheological effects. The purity of the product and the absence therefrom of large particles are essential for a very low abrasivity. Typically the mean particle size should be in the range 0.3 to 1 micron, with a very narrow particle size distribution.
For paper filling calcium carbonate with a mean particle size of 1.5 to 3.0 microns is used.
The average mineral loading for uncoated paper is around 25% by weight while for the coated paper grades it is around 45%
In many polymer, coating and sealant applications, high brightness calcium carbonates with tightly controlled particle sizes are used as functional fillers to bring improvements in appearance, mechanical properties and processing compared with cheap extenders. Typically these products will have an ISO brightness over 90% and a mean particle size of less than 3 microns with a particle top cut of less than 10 microns. By virtue of their purity, fine particle size and lack of large hard particles, these products have relatively low abrasivity so reduce equipment wear in high shear processes such as polymer extrusion.
Calcium carbonate for use in at least some of the above applications may be in a form (so-called GCC) obtained by grinding of naturally occurring calcium carbonate. Alternatively the calcium carbonate can also be produced by a “chemical route” in which carbon dioxide is added to a solution of calcium ions, resulting in precipitation of calcium carbonate, referred to as PCC. Such “chemical routes” can be attractive in that the solution of calcium ions may be generated from a waste lime (CaO) or lime hydroxide (Ca(OH)2) material, thus allowing production of industrially valuable calcium carbonate from a waste material which would otherwise give rise to problems and/or expense for disposal purposes.
A number of industrial processes produce sludges containing calcium carbonate and at least one silicate mineral as a waste product of the process. It has previously been proposed (see below) either to recover calcium carbonate from the sludge or to incinerate the sludge to convert a proportion of the calcium carbonate to calcium oxide which may then be converted to calcium carbonate by a “chemical route”.
Examples of such sludges are those produced by paper mills (and which are referred to as “paper sludge”).
All paper mills use large quantities of water. Typically the solid material content at the start of the process is less than 1%. Much of this water may be recycled but typically losses can be 20 m3 per tonne of paper produced. This water will be charged with fibres and minerals such as calcium carbonate, kaolin and talc and other additives such as starch, latex, optical brightening agents and dyes. The waste water is pumped from the paper machine to a water treatment plant where solid materials are removed by flocculation and sedimentation. In many cases a secondary biological treatment will be used. The resulting sludge is usually then at least partially de-watered, e.g. on a filterpress or in a screw press (in both cases this is mechanical de-watering). This concentrates the sludge to a solid content of typically 60% to 70%. Paper mills using recovered waste paper have a supplementary process where the waste paper is broken down and the fibres are separated from the other materials essentially minerals. All except the fibres will go to the waster water treatment along with the charged water from the paper machine. If the paper mill makes white paper as opposed to grey board the process of separating the fibre and minerals is more sophisticated and uses a de-inking process. The de-inking process separates the cellulose fibres from all the other materials present in the recovered paper using floatation and decantation technology to separate minerals and fibres. However it is not possible to recover 100% of the fibres so a certain quantity of fibres is contained in the waste sludge. Typically for every 100 Tonnes of recovered paper that is recycled, 25 Tonnes of waste sludge will be produced which contains 50-60% organic material. The remaining inorganic material mainly comprises calcium carbonate and kaolin.
Typically the waste from a paper machine might be 3% of the paper produced whereas the waste from a mill using waste paper may be 25% of the paper produced. Thus paper mills using recovered paper as a raw material produce significantly greater quantities of paper sludge that those that do not.
The quantity of waste sludge produced by the European paper industry is several million Tonnes per year.
There are a number of disposal routes for paper sludge including, burning as an alternative fuel in cement kilns, on-site thermal utilisation, composting, land spreading and landfill.
In many countries, more stringent regulations for landfill of organic waste has led to an increase in the burning of paper sludge so now more than half of the sludge in Western Europe is disposed of in this way. This approach enables recovery of thermal energy, elimination of hazardous organic constituents and reduction in waste volume.
Incineration or combustion plants, which meet waste and emission regulations, are designed to extract energy while producing paper sludge ash (PSA). Combustion technologies can include fixed hearth, fluid bed and rotary kilns. The fluid bed combustor (FBC) is often the chosen technology having a high heat transfer efficiency, low capital cost and it can handle aqueous organic sludge.
The composition of PSA typically consists of a mixture of inorganic materials predominately formed from the calcium carbonate and kaolin present in the waste paper sludge. A range of other mineral pigments may also be present in the waste paper sludge including talc, titanium dioxide, calcined clay, bentonite, aluminium trihydrate and precipitated silica.
When the incineration process is controlled at temperatures in the region of 600-800° C. the ash contains a mixture of calcium carbonate, calcium oxide and metakaolin along with some minor amounts of other minerals. In addition some carbon may remain from the burning of the organic constituents
When incineration occurs at temperatures above 800° C. or when the incineration temperature is uncontrolled (often the practice) then most of the calcium carbonate present will decompose to calcium oxide that may react with kaolin and other minor minerals present to form hard glassy calcium aluminium silicate minerals such as gehlenite.
Where the main objective is to recover energy, fluid bed combustors are designed to run at high temperatures (between 800° C. and 1000° C.) but with very short residence times of less than 3 minutes. Under these conditions there is incomplete decomposition of the calcium carbonate and hard glassy silicate minerals may be formed. The incomplete decomposition is probably due to an insufficient time for the adequate transfer of heat into the middle of large agglomerates. Some of the calcium oxide formed immediately reacts with the kaolin and this further depletes the amount of free calcium oxide left in the ash.
In addition some carbon may still remain from the burning of the organic constituents.
Subsequent uses of the ash (PSA) include cement production, lightweight concrete blocks, land spreading and cattle bedding. However PSA has little or no value in these applications. The remaining PSA has traditionally gone to landfill but increasingly this option is discouraged owing to the free lime (Calcium Oxide) content of PSA. PSA is unsuitable for use in blended structural concrete due to the free lime content which will react with atmospheric carbon dioxide so weakening the concrete matrix over a period of time.
There has thus been an incentive to produce and/or recover potentially useful materials from the paper sludge or ash produced by incineration thereof.
Separation of pure fillers from the carbon and/or hard silicate minerals in ash produced during any combustion conditions is extremely difficult. Likewise addition of virgin materials to mask the detrimental effects of un-reacted carbon or hard silicate minerals has not been successful.
In the prior art there are many patent specifications that describe processes for modifying the properties of sludge or PSA in such a way as to make the recycled fillers suitable for paper making but these have failed to achieve a route for recovering a calcium carbonate having both high brightness and low abrasion.
The direct enhancement of deinking sludge is described in EP 0737774 (ECC Int Ltd). In particular the method involves addition of calcium hydroxide to the aqueous sludge mixture followed by carbonation to precipitate calcium carbonate which entrains the ink particles, inorganic pigment particles and organic fibres. This process leads to a reduction in the sludge darkness and increases the amount of calcium carbonate so the sludge may be recycled. This invention suffers from 3 serious drawbacks                1. The majority of the final product is likely to consist of fresh calcium carbonate formed during the recycling process.        2. The enhanced sludge can only be used in very dilute form to supplement virgin fillers used in the manufacture of paper.        3. The ISO brightness of the final product is in the region of 60-70%, still much lower than that of virgin PCC or GCC.        
The final product quality is also significantly influenced by variations in the composition of the deinking sludge.
A similar approach, described in U.S. Pat. No. 5,759,258 (Minerals Technologies, Inc.) and EP 0815175 (Minerals Technologies, Inc.), mentions the addition of calcium hydroxide to the paper sludge ash, produced by high temperature combustion. The objective of the combustion step is to remove the entire organic fraction and decompose the calcium carbonate to calcium oxide. In doing so new stable minerals are formed, including gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8). Calcium hydroxide is added to an aqueous slurry of this ash and carbonated. Carbon dioxide reacts with both the calcium ions originating from the sludge and those introduced as calcium hydroxide to form an outer layer of calcium carbonate around the inner ash particle. Although the product from this invention has a relatively high brightness, this process has serious drawbacks. The majority of the product mass is derived from the added calcium hydroxide; the ISO brightness of 94-96% is insufficient in some paper applications so blending with virgin fillers is required and the product of this invention contains 5-15% of large, hard gehlenite particles which will lead to unacceptably high wire abrasion for use in paper manufacture.
A different approach is adopted in U.S. Pat. No. 5,846,378 (ECC INT Ltd), where carefully controlled combustion of the sludge is employed to optimise the balance of brightness and abrasion. The invention of U.S. Pat. No. 5,846,378 is concerned with removing the organic component while minimising the decomposition of calcium carbonate to calcium oxide. In accordance with the process of U.S. Pat. No. 5,846,378, not more than 50% (and desirably not more than about 25% by weight) of the calcium carbonate is converted to calcium oxide. In this way the formation of hard minerals such as gehlenite is also minimised. A narrow temperature window is specified whereby the fibres and ink burn off leaving a white inorganic fraction mainly consisting of calcium carbonate and metakaolin. Conditions are set to keep the temperature below 800° C. A two stage combustion process is proposed, in order to overcome localised exothermic heating as agglomerated fibres burn. The resultant ash is slaked and carbonated to convert any calcium oxide present to carbonate. This can be followed by intensive grinding to reduce the mineral particle size to that required for the paper making process. The product of this procedure, a mixture of calcium carbonate and metakaolin, has an ISO brightness in the range of 70-75% significantly inferior compared to virgin calcium carbonate and kaolin. It is unsuitable for most applications. The product of the procedure has a relatively high Einlenher wire abrasion, in the region of 30-70 mg. A modification of this process is cited in U.S. Pat. No. 6,063,237 (Imerys) where further calcium hydroxide is added to the ash prior to carbonation, making small improvements in brightness and abrasion. An example in the patent shows that half the product mass derives from this addition of fresh calcium hydroxide.
U.S. Pat. No. 6,830,615 (Imerys) also discloses the controlled temperature combustion approach to manufacture high surface area fillers. In this case the combustion is at higher temperatures (around 800-900° C.) with the purpose of decomposing the maximum amount of calcium carbonate, reacting this with the metakaolin to form hard glassy silicate particles. These are intensively ground to reduce the particle size and abrasivity while unlocking free lime. A carbonation step follows the grinding to prepare a composite filler with high surface area which gives superior opacity in paper. However the wire abrasion is still relatively high and the ISO brightness is in the region of 75-80%, significantly lower than virgin calcium carbonate or calcined kaolin.
In the inventions described above, the paper mill sludge or paper sludge ash is enhanced but there is no separation of the individual minerals, for example the calcium carbonate and kaolin. This means that the end products are dependent on the composition and consistency of the original deinking sludge.
Attempts to separate the minerals present in the sludge have been focussed on the extraction of calcium carbonate using mineral acids. For example, U.S. Pat. No. 7,300,539 (Imerys) describes a route where the deinking sludge is treated with dilute acid which reacts with the calcium carbonate to form calcium salts soluble in water. The calcium salt containing solution is removed from the insoluble fraction and calcium carbonate precipitated by the addition of sodium chloride or sodium hydroxide. The insoluble fraction containing the fibres and predominately kaolin is dried and incinerated at high temperatures to remove organic components and to produce calcined kaolin. The calcium carbonate obtained this way has a slightly superior ISO brightness (83.4%) compared to that obtained by the controlled combustion process above but this is still significantly lower than virgin calcium carbonate.
U.S. Pat. No. 5,919,424 (Thermo Fibergen) cites a similar process with controlled additions of a wide range of acids to either the sludge or the ash. The resulting soluble salts can be extracted and dried but suffer from the drawback of contamination by other metals such as aluminium, magnesium and iron also extracted by the acid.
In summary the attempts to hide the adverse properties of gehlenite and other hard minerals by milling the PSA and forming PCC by precipitation in-situ failed because the abrasion is too high while the milling of the low temperature PSA to produce a fine blended composite filler of calcium carbonate and metakaolin (without the formation of gehlenite and other hard minerals) failed because too much un-burnt carbon was remaining. Therefore the brightness was unsatisfactory.
The other processes described in the prior art utilising dilute acids in order to extract calcium ions from either the sludge or PSA followed by precipitation of calcium carbonate can separate calcium carbonate but only with other metal contaminants. The process economics for this route may only be commercially interesting where the ratio of calcium carbonate to kaolin to low.
There are many patents that include the use of paper mill sludge or PSA in cement and concrete manufacture but only as a waste feed to the cement kiln, along with other waste sludge and ash to help dilute the level of Portland cement.
Other prior art concerns the use of paper mill sludge along with coal ash in compositions suitable for the fabrication of un-fused blocks and aggregates for use in concrete products.
U.S. Pat. No. 5,868,829 (CDEM) relates to a combustion process specifically for the manufacture of a PSA containing a low amount of calcium oxide. Calcium oxide is known to have a detrimental effect on the long term strength of concrete as it will react with carbon dioxide to form calcium carbonate with an increased volume. The reduction of calcium oxide enables the pozzolanic properties of the metakaolin component to be utilised in concrete without the long term weakening of the concrete. This reduction is achieved by controlling the combustion temperature and introducing water into a second combustion chamber to convert the calcium oxide to hydroxide.
US 2005/0223950 (CDEM) discloses a method of treating a material comprising a pozzolanic component to produce a product with enhanced pozzolanic activity. The material to be treated may, for example, be a paper ash containing approximately 30% metakaolin as the pozzolanic component. The ash itself is preferably prepared by thermal treatment of a paper sludge in accordance with the procedure described in PCT/NL95/00280 (equivalent to U.S. Pat. No. 5,868,829—see above). The method of US 2005/0223950 for treating the pozzolanic material (e.g. paper ash) comprises treating the material with an aqueous liquid having a pH of less than 12.5 so as to extract calcium from the material and produce a calcium-enriched aqueous solution and a calcium-depleted solid residue, the latter being the product with enhanced pozzolanic effect. The aqueous liquid used in the treatment process may for example be water but is more preferably an aqueous acidic solution (e.g. hydrochloric acid or acetic acid), optionally containing a chelating compound such as EDTA. The calcium enriched solution is separated from the solid residue which may be used with or without drying to prepare cement or concrete. It is disclosed that the calcium-enriched aqueous solution may be treated with carbon dioxide to produce calcium carbonate but no details of the product quality are given. In the specific Examples of US 2005/0223950 two materials are treated in accordance with the procedure of the patent specification to provide a product of enhanced pozzolanic activity. One material is designated CDEM-ash 1 which is stated to be obtained by thermal treatment of paper ash as described in PCT/NL95/00280 (equivalent to U.S. Pat. No. 5,868,829—see above). The other material, designated as CDEM-ash 2 is obtained by heating CDEM-ash 1 in air in an electric furness at 1050° C. for 12 hours. The mineral composition of CDEM-ash 1 as determined by X-ray diffraction shows it to have a CaCO3:CaO ratio of 71:29 which represents about 41% conversion of the calcium carbonate in the original paper sludge. The incineration conditions used for producing CDEM-ash 2 would result in a considerable amount of calcium oxide reacting to form silicates and aluminates.